Generally, a metal can is coated in order to prevent corrosion of the inner and outer surfaces. In recent years, for the purposes of simplifying the process, improving hygiene, preventing pollution, etc., a method in which a metal can is covered with a thermoplastic resin film, such as a polyester film, has been performed for imparting rust resistance without using an organic solvent. That is, a thermoplastic resin film is laminated to a metal plate of tin, tin-free steel, aluminum, or the like, and then used for food can, beverage can, or aerosol can applications in which cans are subjected to severe forming processing, such as in the case of drawn cans and thinned drawn cans. In terms of cost reduction, cans for these applications have been produced through thinning drawing and ironing under even severer processing conditions.
In the case where such severe forming processing is performed, with the thinning of the metal plate, the resin film is also thinned. Generally, in order to improve the design features, the outer surface of a food can or a beverage can is printed. In a can formed from a resin film-covered metal plate, a resin film containing white or various color pigments is laminated to the metal plate, and the laminated is used as the printing substrate in order to conceal the color of the metal plate. In the case where such a laminate metal plate is subjected to severe processing, the resin thickness significantly decreases, whereby the absolute quantity of the added pigment in the thickness direction decreases, resulting in a problem in that the substrate does not exhibit sufficient concealability. In the case where a large amount of pigment is previously added to the resin film with forethought of this problem, the strength of the resin film decreases. Accordingly, the resin film is likely to be chipped or scratched during processing, and it further happens that the resin film cracks and falls off. Thus, it is difficult to improve the concealability while keeping the strength of the covering resin film high.
For example, a method in which a biaxially stretched polyester film is laminated to a metal plate, and the laminate is used as a material for can manufacturing, has been proposed (JP-A-11-342577, JP-A-2000-37836). However, when forming is performed through severer processing, the resin film is chipped or scratched, or may break in an extreme case. In addition, a method in which an unstretched polyester film is laminated to a metal plate, and the laminate is used as a material for can manufacturing, has been proposed (JP-A-11-348218). However, unstretched films are extremely brittle and likely to be cut during film production or handling, leading to the problem of poor productivity.
In order to solve these problems, WO 2013/002323 proposes a colored biaxially stretched laminate film including a surface layer made of a high-polymerization-degree copolyester and a back layer made of a copolyester having a high concentration of colorant. With this film, it is certainly possible to achieve both concealability and the strength of the resin film.
However, in recent years, for the purposes of relaxing the stress accompanying the strain applied to the film of a laminate metal plate, thereby reducing damage to the film in the subsequent forming processing, a heat treatment is performed in the course of a series of can manufacturing steps. In the heat treatment, in order to balance with the enamel rater value that indicates the presence/absence of coating defects in a polyester film for covering the can inner surface, a higher heat treatment temperature has been required. However, this causes appearance defects in the film as another problem, and there has been a demand for its resolution.